1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly, to an image forming apparatus and an image forming method which are used to generate an image of an object with an electromagnetic wave including at least one frequency of a frequency region from 30 GHz to 30 THz (referred to as a terahertz (THz) wave in this specification).
2. Description of the Related Art
Up to now, a method of generating a terahertz image of an object based on a signal of a terahertz wave has been proposed (see Japanese Patent Application Laid-Open No. H08-320254). According to the method, the terahertz wave is focused on a specific region of the object. The object is moved such that the focus point passes through each of multiple spatially separated regions of the object. Respective transmitted signals propagating through the spatially separated points of the object are collected. The signals are processed to generate the image of the object.
In the case of imaging using the method, in order to increase the intensity of the terahertz wave to improve an S/N ratio, the terahertz wave is focused on a single point of the object. However, when the terahertz wave is focused on the single point, it is necessary to scan all the regions of the object with a point beam, so the imaging takes a long period of time.
Therefore, Japanese Patent Application Laid-Open No. H08-320254 describes a method involving irradiating the entire object with a terahertz beam and focusing the beam that has passed therethrough on a focal plane THz detector array by a lens system to perform imaging at once. However, the focal plane THz detector array is a two-dimensional array of THz dipole antennas, so a lock-in amplifier is required for each element to perform the imaging. Because the lock-in amplifier is a device which requires a space, when as many lock-in amplifiers as the number of elements used for the focal plane THz detector array are prepared, an apparatus becomes very large in size. Thus, methods of easily performing imaging using a small-size apparatus in a short time have been expected.
A method using, as a terahertz wave detector, a pyroelectric sensor array which operates at normal temperature and has a small size has been studied as one of the methods of easily performing imaging using a small-size apparatus in a short time. The pyroelectric sensor array is sensitive to infrared light, but the sensitivity to the terahertz wave is not sufficient and the response characteristic is insufficient, so long-time averaging is performed to increase the S/N ratio. Therefore, according to the method, it is difficult to perform short-time imaging.
An imaging system and an imaging method which are used for imaging with the terahertz wave have been proposed (see Japanese Patent Application Laid-Open No. 2006-508333). In the imaging method, an interference pattern is generated using multiple terahertz wave sources and a signal thereof is detected by an interferometer array including multiple spaced detectors. A Fourier component on the Fourier-transform plane is produced based on the detected signal and then inverse-Fourier-transformed for image reconstruction. However, the method provides a spatial THz imaging technique capable of simultaneously detecting THz waves from the multiple THz wave sources within a wide “field of view”, so the multiple THz wave sources are necessary.